Torrefaction - Biomass Gasification, Pyrolysis and Torrefaction - page 98

4.4.1.3 Postdrying Heating
After the biomass is dried, it needs to be heated further to the designed torrefac-
tion temperature, T t . This temperature is generally in excess of 200 Cbecause
very little decomposition of the biomass takes place below this temperature.
During this stage, all physically bound moisture along with some light
organic compounds escape from the biomass (Bergman et al., 2005). One
can see in Figure 4.3 that the energy demand of this stage is relatively small
because it provides only sensible heat to the dried biomass.

M f ð 1

2 M Þ C pd ð T t 2

100 Þ

Energy required ; Q pdh 5 (4.3)

h u ; pdh

where C pd is the specific heat of dry biomass and h u,pdh is the heat utilization
efficiency of this section.

4.4.1.4 Torrefaction
Torrefaction stage is key to the whole process as the bulk of depolymeriza-
tion of the biomass takes place in this stage. A certain amount of time is
needed to allow the desired degree of depolymerization of the biomass to
occur. The degree of torrefaction depends on the reaction temperature as
well as on the time the biomass is subjected to torrefaction. This time is also
called reactor residence time or torrefaction time.
The torrefaction time should be measured from the instant the biomass
reaches the temperature for the onset of torrefaction (200 C) because the deg-
radation of biomass below this temperature is negligible. The torrefaction pro-
cess is mildly exothermic (Prins, 2005) over the temperature range of
250 300 C. So, except for heat loss, the torrefaction stage should require

very little energy ( Figure 4.3 ), but in practice it could require some heat to
make up for the unavoidable heat loss from the torrefaction section of the
reactor.

Q tor 5 H loss 1 M f ð 1 M Þ X t (4.4)

Here, X t is a parameter (kJ/kg product) that determines the amount of
heat absorbed during torrefaction. It is positive for endothermic and negative
for exothermic torrefaction reactions. The amount of heat loss H loss to the
ambience from the torrefaction section is a function of reactor design.

4.4.1.5 Cooling
Biomass leaves the torrefier at the torrefaction temperature, which is the
highest temperature in the system. This being generally above the ignition
temperature of most torrefied biomass (Table 3.8), unless cooled down suffi-
ciently the product could catch fire on contact with air. Additionally, handling
of such a hot product is unsafe and dangerous. So, the torrefied product must
be cooled down from the torrefaction temperature (T t ), to acceptable